The invention relates to methods for processing clays, shales and other similar clay ceramic materials containing natural sulfide contaminants, such as pyrite. In particular, the invention relates to improved methods for removing such contaminants from clay products.
Natural clays and other materials that are mined for the production of bricks, tiles, and ceramic products may contain natural sulfides. Such sulfides may include pyrite, also known as iron sulfide FeS2. Sulfur-containing compounds such as pyrite are typically removed from ceramic and brick products during manufacture by thermal treatment during firing of the product. This treatment requires special cycles and extended process times, and typically generates acid gas emissions from the kiln.
Acid gasses such as sulfur dioxide (i.e.: SO2), sulfur trioxide (ie: SO3) or sulfuric acid (i.e: H2SO4) can be produced in the kiln, although sulfur dioxide appears to be the most common product of sulfide oxidation. If these sulfur-containing compounds (i.e. contaminants) are not driven from the products during firing, surface defects such as efflorescence may occur in the products at a later date.
Brick and other structural clay products such as quarry tile and roofing tile are nearly 100 percent clay content. Such materials have the characteristics of plasticity, and offer and a reasonable firing range temperature. The majority of brick made in North America are produced by stiff extrusion, followed by drying and firing. The firing step of brick production generally includes processing temperatures in the xe2x80x9cred heatxe2x80x9d range of 950-1250xc2x0 C. The products are heated to such temperatures on a predetermined schedule.
There has been a need in the industry to provide a manufacturing method which can reduce undesirable efflorescence. Efflorescence may be described as an undesirable deposit or xe2x80x9cblooming outxe2x80x9d of substances from the interior onto the surface of masonry or brick which discolors the surface and detracts from the appearance of the structure. Efflorescence substances may comprise salts, acid, bases or colloidal gels.
In the past, techniques reducing the effects of efflorescence in making ceramic products have included maintaining a kiln temperature of about 1000xc2x0 C. and flooding with oxygen to completely oxidize the clay during firing. However, such firing for extended periods of time, and elevated temperatures, is time consuming and expends a significant amount of energy.
Sulfur dioxide (SO2) is known as a xe2x80x9ccriteria pollutantxe2x80x9d that is subject to regulation by state and federal environmental authorities. During thermal processing or xe2x80x9cfiringxe2x80x9d, all of the SO2 released due to oxidation is not immediately released. In fact, the clay holds some as xe2x80x9csulfatexe2x80x9d like phases with further liberation occurring during a soak period. In general, it has been shown that once the residual sulfur content in a fired brick reaches a characteristic threshold, then sulfuric acid will form in the brick once it is in a wall in a sufficient concentration to cause undesirable efflorescence. Thus, it is important to remove sulfur-containing compounds from brick during the brickmaking process.
One of the disadvantages of conventional ceramic processes is that they require large amounts of energy to maintain sufficiently high temperatures for a sufficient length of time in order to completely remove sulfur-containing compounds from the brick. Most processes require fairly extended process times, and many brickmaking processes lead to the emission of undesirable acid gasses, such as sulfur dioxide in the kiln.
What is needed in the industry is a method to effectively remove sulfur-containing contaminants by firing with reduced processing times, resulting in energy savings and enhanced output. A method that would facilitate removal or degradation of sulfur-containing compounds from such products prior to firing in the kiln may result in cleaner emissions from the kiln exhaust. Thus, a method that is capable of reducing the incidence of efflorescence in clay products and masonry by ensuring complete removal of pyrite from the product in a more effective manner would be highly desirable.
A method of treating clay, shale and other ceramic materials containing pyrite is provided in the invention. In the manufacture of brick, the particle size of mined clay, shale or other ceramic raw material is reduced. For purposes of this description, xe2x80x9cclayxe2x80x9d shall refer to clay, shales, and other natural or synthetic brickmaking material. Then, the clay is mixed with an oxidizer to disperse the oxidizer within the clay to expose the maximum amount of clay surface to the oxidizer. The clay is shaped into clay products (i.e.: bricks, tiles or the like) and heated to elevated temperatures. The pyrite within the clay is oxidized by the action of the oxidizer, thereby removing sulfur-containing compounds from the clay. The application of the invention may assist in preventing efflorescense by ensuring complete or nearly complete removal of pyrite from the product through the combination of oxidation treatment and subsequent firing.
Many different oxidizers may be employed in the practice of the invention, as further provided in the description below. In one preferred application of the invention, the oxidizer employed comprises a solution of hydrogen peroxide. In other embodiments, an optional step of aging the clay in air is provided.